8. Elimination Reactions

When 2-bromo-3-phenylbutane is treated with sodium methoxide, two alkenes result (by E2 elimination). The Zaitsev product predominates. b. When one pure stereoisomer of 2-bromo-3-phenylbutane reacts, one pure stereoisomer of the major product results. For example, when (2R,3R)-2-bromo-3-phenylbutane reacts, the product is the stereoisomer with the methyl groups cis. Use your models to draw a Newman projection of the transition state to show why this stereospecificity is observed. c. Use a Newman projection of the transition state to predict the major product of elimination of (2S,3R)-2-bromo-3-phenylbutane. d. Predict the major product from elimination of (2S,3S)-2-bromo-3-phenylbutane. This prediction can be made without drawing any structures, by considering the results in part (b).

704

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What is the major elimination product obtained from the reaction of each of the following alkyl halides with hydroxide ion? c.

396

Why do cis-1-bromo-2-ethylcyclohexane and trans-1-bromo-2-ethylcyclohexane form different major products when they undergo an E2 reaction?

243

For each of the following compounds, draw the product that forms in an E2 reaction and indicate its configuration: b. (1S,2R)-1-bromo-1,2-diphenylpropane

315

When the following reactions are carried out under the same conditions, the rate constant for the first reaction (kH) is found to be 7 times greater than the rate constant for the second reaction (kD). What does that tell you about the mechanism of the reaction? (Hint: a C—D bond is 1.2 kcal/mol stronger than a C—H bond.)

181

cis-4-Bromocyclohexanol and trans-4-bromocyclohexanol form the same elimination product but a different substitution product when they react with HO−.

a. Why do they form the same elimination product?

265

When 2-bromo-2,3-dimethylbutane reacts with a strong base, two alkenes (2,3-dimethyl-1-butene and 2,3-dimethyl-2-butene) are formed. a. Which of the bases (A, B, C, or D) would form the highest percentage of the 1-alkene? b. Which would give the highest percentage of the 2-alkene?

633

Draw the elimination products for each of the following E2 reactions; if the products can exist as stereoisomers, indicate which stereoisomers are obtained. d. (2R,3R)-2-chloro-3-methylpentane + high concentration of CH3O-

218

Why is a cumulated diene not formed in the reaction shown above?

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a. Explain why 1-bromo-2,2-dimethylpropane has difficulty undergoing both SN2 and SN1 reactions. b. Can it undergo E2 and E1 reactions?

628

Explain why only a substitution product and no elimination product is obtained when the following compound reacts with sodium methoxide:

268

a. What is the major product obtained when each of the following compounds undergoes an E2 reaction with methoxide ion? Show the configuration of the product. b. Does the product obtained depend on whether you start with the R or S enantiomer of the reactant? 1.

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For each of the following alkyl halides, indicate the stereoisomer that would be obtained in greatest yield in an E2 reaction. b. 4-bromo-2,2,3,3-tetramethylpentane

314

For each of the following alkyl halides, indicate the stereoisomer that would be obtained in greatest yield in an E2 reaction. a. 3-bromo-2,2,3-trimethylpentane

184

Rank the following from most reactive to least reactive in an E2 reaction:

270

Which reactant in each of the following pairs undergoes an elimination reaction more rapidly? Explain your choice. a. (CH3)3CCl→CH3O− or (CH3)3CBr→CH3O− b.

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Which isomer reacts more rapidly in an E2 reaction: cis-1-bromo-4-tert-butylcyclohexane or trans-1-bromo-4-tert-butylcyclohexane? Explain your answer.

236

Which alkyl halide in each pair is more reactive in an E2 reaction with hydroxide ion? c.

305

What products will be obtained from the E2 reaction of the following alkyl halides? c.

186

What is the major elimination product obtained from an E2 reaction of each of the following alkyl halides with hydroxide ion? f.

228

What is the major elimination product obtained from an E2 reaction of each of the following alkyl halides with hydroxide ion? e.

294

Draw the elimination products for each of the following E2 reactions; if the products can exist as stereoisomers, indicate which stereoisomers are obtained. c. (2R,3S)-2-chloro-3-methylpentane + high concentration of CH3O-

331

Two elimination products are obtained from the following E2 reaction: CH3CH2CHDCH2Br→CH3O− a.What are the elimination products? b. Which is formed in greater yield? [HINT: SEE PROBLEM 9-91]

285

What is the major elimination product obtained from an E2 reaction of each of the following alkyl halides with hydroxide ion? a.

261

What is the major elimination product obtained from an E2 reaction of each of the following alkyl halides with hydroxide ion? b.

149

What products will be obtained from the E2 reaction of the following alkyl halides? a.

167

What products will be obtained from the E2 reaction of the following alkyl halides? b.

165

Draw the major product obtained when each of the following alkyl halides undergoes an E2 reaction: a.

196

Draw the major product obtained when each of the following alkyl halides undergoes an E2 reaction: b.

149

Draw the major product obtained when each of the following alkyl halides undergoes an E2 reaction: e.

141

Draw the major product obtained when each of the following alkyl halides undergoes an E2 reaction: f.

162

When 2-bromopropane is treated with sodium ethoxide, propene is produced. What molecule is lost from 2-bromopropane in this process?

892

Justify on a reaction coordinate diagram the fact that a strong base like sodium amide (NaNH₂) results in a faster E2 elimination than does sodium hydroxide (NaOH).

120

The cis-diastereomer undergoes E2 elimination 500 times faster than the trans form. Explain.

124

Practice your electron-pushing skills by drawing a mechanism for the following E2 reactions. (a)

139

(•) Suggest a mechanism for the following elimination reactions. (c)

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(••) Predict the major product(s) of the following elimination reactions, paying close attention to the stereochemical outcome of the reactions. (a)

151

When a secondary haloalkane is treated with sodium ethoxide in ethanol, we predict alkene formation over ether formation. How did we make this determination?

207

What products would you expect to obtain from the following reactions? c. urea+water d. beta-ethylglutaric acid + acetyl chloride + Δ

219

What are the products of the following reactions? g. + CH3OH excess CH3O- —> h. (CH3C=O)2O Δ—>

100

Compare each of the mechanisms listed here with the mechanism for each of the two parts of the acid-catalyzed hydrolysis of an ester, indicating a. similarities. b. differences. 4. acid-catalyzed hydrolysis of an amide (Section 15.12)

150

Which compound has the greatest rate of hydrolysis at pH = 3.5: benzamide, o-carboxybenzamide, o-formylbenzamide, or o-hydroxybenzamide?

103

Practice your electron-pushing skills by drawing a mechanism for the following E2 reactions.

(b) <IMAGE>

(••) Predict the major product(s) of the following elimination reactions, paying close attention to the stereochemical outcome of the reactions.

(b) <IMAGE>

Identify the bonds that break and form in the following elimination reactions.

(b) <IMAGE>

If a quaternary ammonium ion can undergo an elimination reaction with a strong base, why can’t a protonated tertiary amine undergo the same reaction?

b. Indicate how each of the same factors affects an E2 reaction.

1. the strength of the base

2. the concentration of the base

3. the solvent

Draw the elimination products for each of the following E2 reactions; if the products can exist as stereoisomers, indicate which stereoisomers are obtained.

e. 3-chloro-3-ethyl-2,2-dimethylpentane + high concentration of CH3CH2O-

a. What is the major product obtained when each of the following compounds undergoes an E2 reaction with methoxide ion? Show the configuration of the product.

b. Does the product obtained depend on whether you start with the R or S enantiomer of the reactant?

3. <IMAGE>

Which alkyl halide in each pair is more reactive in an E2 reaction with hydroxide ion?

a. <IMAGE>

b. <IMAGE>

Which alkyl halide in each pair is more reactive in an E2 reaction with hydroxide ion?

a. <IMAGE>

b. <IMAGE>

Which alkyl halide in each pair is more reactive in an E2 reaction with hydroxide ion?

c. <IMAGE>

d. <IMAGE>

cis-1-Bromo-4-tert-butylcyclohexane and trans-1-bromo-4-tert-butylcyclohexane both react with sodium ethoxide in ethanol to form 4-tert-butylcyclohexene. Explain why the cis isomer reacts much more rapidly than the trans isomer.

105

Draw the structure of DDE.

For each of the following alkyl halides, indicate the stereoisomer that would be obtained in greatest yield in an E2 reaction.

c. 3-bromo-2,3-dimethylpentane

d. 3-bromo-3,4-dimethylhexane